Method for making electrode active material

ABSTRACT

The present invention is directed to a method for making electrode active materials represented by the general formula:
 
A a (VO) b XO 4 ,
         wherein:   (a) A is an alkali metal or mixture of alkali metals, and 0&lt;a&lt;4;   (b) 0&lt;b&lt;2;   (c) X is selected from the group consisting of phosphorous (P), sulfur (S), arsenic (As), silicon (Si), and combinations thereof; and
 
wherein A, X, a and b are selected to maintain the electroneutrality of the electrode active material in its nascent (as prepared or synthesized) state.

FIELD OF THE INVENTION

This invention relates to a novel method for synthesizing electrodeactive materials for use in secondary electrochemical cells.

BACKGROUND OF THE INVENTION

A battery pack consists of one or more secondary (rechargeable)electrochemical cells or batteries, wherein each cell typically includesa positive electrode, a negative electrode, and an electrolyte or othermaterial for facilitating movement of ionic charge carriers between thenegative electrode and positive electrode. As the cell is charged,cations migrate from the positive electrode to the electrolyte and,concurrently, from the electrolyte to the negative electrode. Duringdischarge, cations migrate from the negative electrode to theelectrolyte and, concurrently, from the electrolyte to the positiveelectrode.

Compounds of the formula VOXO₄ (X=S, P or As) are known to be suitablefor use as electrode active materials in lithium anode-based secondaryelectrochemical cells (Li/VOXO₄). However, these materials must belithiated in order to make these materials useful in a lithium-ion cell(e.g. a cell containing a graphitic negative electrode). Lithiation ofthese electrode active materials can be accomplished chemically usinglithiating agents such as butyl lithium and lithium iodide. However,such chemical lithiation techniques are expensive, slow and produce lowyields, therefore making them unsuitable for commercial use.

Accordingly, there is a current need for a method of making compounds ofthe formula AVOXO₄ (A=an alkali metal, and X=S, P or As), which iseasier, faster, less expensive and/or produces greater yields, thanthose methods known in the art.

SUMMARY OF THE INVENTION

The present invention is directed to a novel method for making electrodeactive materials represented by the general formula:A_(a)(VO)_(b)XO₄,

-   -   wherein:    -   (a) A is an alkali metal or mixture of alkali metals and 0<a<4;    -   (b) 0<b<2;    -   (c) X is selected from the group consisting of phosphorous (P),        sulfur (S), arsenic (As), silicon (Si), and combinations        thereof; and    -   (d) A, X, a and b are selected to maintain the electroneutrality        of the electrode active material in its nascent (as prepared)        state;

wherein the method includes the step of precipitating the compound froman aqueous solution containing vanadyl ions (VO²⁺), alkali metal ionsand ions of moiety XO₄.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional diagram illustrating the structureof a non-aqueous electrolyte cylindrical electrochemical cell of thepresent invention.

FIG. 2 is a schematic cross-sectional diagram illustrating the structureof another embodiment of an electrochemical cell of the presentinvention.

FIG. 3 shows the results of an x-ray powder diffraction analysis of afirst sample of LiVOPO₄ prepared according to the invention, as well asan analysis of the electrochemically delithiated form of the firstsample.

FIG. 4 is a plot of cathode specific capacity vs. cell voltage for afirst Li/1M LiPF₆ (EC/DMC)/LiVOPO₄ cell.

FIG. 5 shows the results of an x-ray powder diffraction analysis of asecond sample of LiVOPO₄ prepared according to the invention, bothbefore and after dehydration.

FIG. 6 is an EVS differential capacity plot for a second Li/1M LiPF₆(EC/DMC)/LiVOPO₄ cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has been found that the novel methods of this invention affordsbenefits over such methods among those known in the art. Such benefitsinclude, without limitation, one or more of ease of manufacture, greateryields, faster reaction times, and reduced costs. Specific benefits andembodiments of the present invention are apparent from the detaileddescription set forth herein below. It should be understood, however,that the detailed description and specific examples, while indicatingembodiments among those preferred, are intended for purposes ofillustration only and are not intended to limit the scope of theinvention.

The present invention is directed to a method for making electrodeactive materials represented by the general formula (I):A_(a)(VO)_(b)XO₄,  (I)

-   -   wherein:    -   (a) A is an alkali metal or mixture of alkali metals, and 0<a<4;    -   (b) 0<b<2;    -   (c) X is selected from the group consisting of phosphorous (P),        sulfur (S), arsenic (As), silicon (Si), and combinations        thereof; and

wherein A, X, a and b are selected to maintain the electroneutrality ofthe electrode active material in its nascent (as prepared orsynthesized) state.

Unless otherwise specified, a variable described herein algebraically asequal to (“=”), less than or equal to (“≦”), or greater than or equal to(“≧”) a number is intended to subsume values or ranges of values aboutequal or functionally equivalent to said number.

As referred to herein “electroneutrality” is the state of the electrodeactive material wherein the sum of the positively charged species (e.g.,A and moiety (VO)) in the material is equal to the sum of the negativelycharged species (e.g., XO₄) in the material. Preferably, the XO₄ moietyis an anion having a charge of −2, −3, or −4, depending on the selectionof X. Upon the selection of X for moiety XO₄, selection of the valuesfor variables a (moiety A) and b (moiety (VO)) are governed by theformula (A):a+2b=V^(XO4),  (A)wherein V^(XO4) is the oxidation state for moiety XO₄.

Electrode active materials represent by general formula (I), made by themethods described herein, are characterized has having VOXO₄ host layerscomposed of corner-sharing VO₆ octahedra and XO₄ tetrahedra,intercalated with guest species (A) between the host layers.

For all embodiments described herein, A is selected from the groupconsisting of lithium (Li), sodium (Na), potassium (K), and mixturesthereof. In one subembodiment, A is selected from the group consistingof Li, a mixture of Li with Na, a mixture of Li with K, and a mixture ofLi, Na and K. In another subembodiment, A is Li. In anothersubembodiment, A is Na. For all embodiments herein, 0<a<4. In onesubembodiment, 0<a≦2. In one subembodiment, a=1. In anothersubembodiment, a=2.

For all embodiments described herein, 0<b<2. In one subembodiment,0<b≦1. In another subembodiment, 0<b≦0.5. In another subembodiment, b=1.

For all embodiments described herein, X is selected from the groupconsisting of phosphorous (P), sulfur (S), arsenic (As), silicon (Si),and combinations thereof. In one subembodiment, X is P. In anothersubembodiment, X is selected from the group consisting of Si, P, andmixtures thereof. In another subembodiment, X=(Si_(1-x)P_(x)), wherein0<x<1.

As used herein, the recitation of a genus of elements, materials orother components, from which an individual component or mixture ofcomponents can be selected, is intended to include all possiblesub-generic combinations of the listed components, and mixtures thereof.

Electrode active materials represent by general formula (I) are preparedby precipitating the electrode active material from an aqueous solutioncontaining vanadyl ions (VO²⁺), alkali metal ions and ions of moietyXO₄.

In one embodiment, the electrode active materials represent by generalformula (I) are prepared by first preparing an aqueous solutioncontaining one or more ion species selected from the group consisting ofvanadyl ions, alkali metal ions and ions of moiety XO₄. The aqueoussolution is prepared by dissolving one or more vanadyl precursorcompounds, one or more alkali metal precursor compounds, and/or one ormore precursor compounds containing moiety XO₄, in an aqueous solvent.In one embodiment, one or more alkali metal precursor compounds and theone or more precursor compounds containing moiety XO₄, are the same.Stated differently, the alkali metal ions and moiety XO₄ derive from thesame compound. In another embodiment, the alkali metal ions are presentin solution with either the vanadyl ions or the ions of moiety XO₄,wherein the third ion species is then added and the electrode activematerial is formed by precipitation.

The precursor compounds can be dissolved separately and then combined toform a single aqueous solution, dissolved simultaneously in one step(single step dissolution reaction), or dissolved using a step-wisedissolution reaction (e.g. by dissolving the first precursor in anaqueous solution, followed by the addition and dissolution of thesecond, and so forth until all the requisite precursors have beendissolved and the precipitant material forms).

During the precipitation step, the aqueous solution is acidic to assistin complete dissolution of the vanadyl ions. In one subembodiment, pH≦5at standard temperature and pressure or “STP” (0° C. and 1 atmosphere(atm) of absolute pressure) In one subembodiment, pH≦4 at STP. Inanother subembodiment, 0≦pH≦5 at STP. In another subembodiment, 0≦pH≦4at STP. In yet another subembodiment, 2≦pH≦4.

Vanadyl precursor compounds useful herein for preparing an aqueoussolution containing vanadyl ions include V₂O₅, V₂O₃, VOSO₄, VOC₂O₄, andmixtures thereof. Alkalized forms of the aforementioned precursorcompounds may also be used.

Alkali metal precursor compounds useful herein for preparing an aqueoussolution containing alkali metal ions include AOH, A₂CO₃, A₃C₆H₅O₇,ANO₃, CH₃AO, Al, ACl, and mixtures thereof, wherein A is an alkalimetal. Precursor compounds useful herein for preparing an aqueoussolution containing alkali metal ions as well as one of the other twoion species include A₂SO₄, A₄SiO₄, AH₂PO₄, A₃PO₄, HAsA₂O₄, and mixturesthereof, wherein A is an alkali metal.

Precursor compounds containing moiety XO₄ which are useful herein forpreparing an aqueous solution containing ions of moiety XO₄ includeH₃PO₄, H₂SO₄, and mixtures thereof.

The precursor compounds selected preferably disassociate in the aqueoussolution to produce counter-ions (e.g. Cl⁻, I⁻, and SO₄ ²⁻) which do notcompete with the precipitation of the electrode active material or forminsoluble impurities which could otherwise become entrained within theelectrode active material precipitant.

Where two of the three ions participating in the formation of theelectrode active material tend to form other compounds and thereforecompete with the precipitation of the electrode active material, stepsshould be taken to avoid formation of those other, unwanted compounds toensure the electrode active material is substantially pure. For example,in one embodiment where alkali metal ions (e.g. Li⁺) and phosphate ions(PO₄ ³⁻) are present in solution, alkali metal phosphates (e.g. A₃PO₄)may form, typically when pH is greater than 5 at STP. Formation of A₃PO₄can be substantially avoided by conducting the precipitation reaction inan acidic media buffered by an acidic buffer and/or by excess acidicprecursor. Preferably, the acidic buffer yields ions that do not competewith or participate in, the precipitation reaction. Suitable buffersinclude HCl, HNO₃, H₂O₂, and combinations thereof.

If an aqueous solution containing one or two ion species selected fromthe group consisting of vanadyl ions, alkali metal ions and ions ofmoiety XO₄ is prepared, the remaining ion species are subsequently added(preferably after the pH has been adjusted, if necessary, in order tosubstantially avoid competing reactions). Thereafter, the solution isstirred and the active material precipitate is collected.

The present invention also provides a secondary electrochemical cellcontaining an electrode active material made by the method describedherein. The present invention also provides for batteries containing thenovel electrode active material described by general formulas (I), (IV)and (V), wherein the battery includes:

-   -   (a) a first electrode (also commonly referred to as a positive        electrode or cathode) which includes an active material of the        present invention;    -   (b) a second electrode (also commonly referred to as a negative        electrode or anode) which is a counter-electrode to the first        electrode; and    -   (c) an electrolyte in ion-transfer communication with the first        and second electrodes.

The electrode active material of this invention may be incorporated intothe first electrode, the second electrode, or both. Preferably, theelectrode active material is employed in the cathode. The architectureof a battery of the present invention is selected from the groupconsisting of cylindrical wound designs, wound prismatic and flat-plateprismatic designs, and polymer laminate designs.

Referring to FIG. 1, in one embodiment, a novel secondaryelectrochemical cell 10 having an electrode active material of thepresent invention, includes a spirally coiled or wound electrodeassembly 12 enclosed in a sealed container, preferably a rigidcylindrical casing 14 as illustrated in FIG. 1. In one subembodiment,the cell 10 is a prismatic-type cell, and the casing has a substantiallyrectangular cross-section (not illustrated).

Referring again to FIG. 1, the electrode assembly 12 includes: apositive electrode 16 consisting of, among other things, an electrodeactive material represented by general formula (I); a counter negativeelectrode 18; and a separator 20 interposed between the first and secondelectrodes 16,18. The separator 20 is preferably an electricallyinsulating, ionically conductive microporous film, and composed of apolymeric material selected from the group consisting of polyethylene,polyethylene oxide, polyacrylonitrile and polyvinylidene fluoride,polymethyl methacrylate, polysiloxane, copolymers thereof, andadmixtures thereof.

Each electrode 16,18 includes a current collector 22 and 24,respectively, for providing electrical communication between theelectrodes 16,18 and an external load. Each current collector 22,24 is afoil or grid of an electrically conductive metal such as iron, copper,aluminum, titanium, nickel, stainless steel, or the like, having athickness of between 5 μm and 100 μm, preferably 5 μm and 40 μm.Optionally, the current collector may be treated with an oxide-removingagent such as a mild acid and the like, and coated with an electricallyconductive coating for inhibiting the formation of electricallyinsulating oxides on the surface of the current collector 22,24.Examples of a suitable coatings include polymeric materials comprising ahomogenously dispersed electrically conductive material (e.g. carbon),such polymeric materials including: acrylics including acrylic acid andmethacrylic acids and esters, including poly (ethylene-co-acrylic acid);vinylic materials including poly(vinyl acetate) and poly(vinylidenefluoride-cohexafluoropropylene); polyesters including poly(adipicacid-co-ethylene glycol); polyurethanes; fluoroelastomers; and mixturesthereof.

The positive electrode 16 further includes a positive electrode film 26formed on at least one side of the positive electrode current collector22, preferably both sides of the positive electrode current collector22, each film 26 having a thickness of between 5 μm and 150 μm,preferably between 25 μm an 125 μm, in order to realize the optimalcapacity for the cell 10. The positive electrode film 26 is composed ofbetween 80% and 95% by weight of an electrode active materialrepresented by general formula (I), between 1% and 10% by weight binder,and between 1% and 10% by weight electrically conductive agent.

Suitable binders include: polyacrylic acid; carboxymethylcellulose;diacetylcellulose; hydroxypropylcellulose; polyethylene; polypropylene;ethylenepropylene-diene copolymer; polytetrafluoroethylene;polyvinylidene fluoride; styrene-butadiene rubber;tetrafluoroethylene-hexafluoropropylene copolymer; polyvinyl alcohol;polyvinyl chloride; polyvinyl pyrrolidone;tetrafluoroethyleneperfluoroalkylvinyl ether copolymer; vinylidenefluoride-hexafluoropropylene copolymer; vinylidenefluoride-chlorotrifluoroethylene copolymer; ethylenetetrafluoroethylenecopolymer; polychlorotrifluoroethylene; vinylidenefluoride-pentafluoropropylene copolymer; propylene-tetrafluoroethylenecopolymer; ethylene-chlorotrifluoroethylene copolymer; vinylidenefluoridehexafluoropropylene-tetrafluoroethylene copolymer; vinylidenefluorideperfluoromethylvinyl ether-tetrafluoroethylene copolymer;ethylene-acrylic acid copolymer; ethylene-methacrylic acid copolymer;ethylene-methyl acrylate copolymer; ethylene-methyl methacrylatecopolymer; styrene-butadiene rubber; fluorinated rubber; polybutadiene;and admixtures thereof. Of these materials, most preferred arepolyvinylidene fluoride and polytetrafluoroethylene.

Suitable electrically conductive agents include: natural graphite (e.g.flaky graphite, and the like); manufactured graphite; carbon blacks suchas acetylene black, Ketjen black, channel black, furnace black, lampblack, thermal black, and the like; conductive fibers such as carbonfibers and metallic fibers; metal powders such as carbon fluoride,copper, nickel, and the like; and organic conductive materials such aspolyphenylene derivatives.

The negative electrode 18 is formed of a negative electrode film 28formed on at least one side of the negative electrode current collector24, preferably both sides of the negative electrode current collector24. The negative electrode film 28 is composed of between 80% and 95% ofan intercalation material, between 2% and 10% by weight binder, and(optionally) between 1% and 10% by weight of an electrically conductiveagent.

Intercalation materials suitable herein include: transition metaloxides, metal chalcogenides, carbons (e.g. graphite), and mixturesthereof. In one embodiment, the intercalation material is selected fromthe group consisting of crystalline graphite and amorphous graphite, andmixtures thereof, each such graphite having one or more of the followingproperties: a lattice interplane (002) d-value (d₍₀₀₂₎) obtained byX-ray diffraction of between 3.35 Å to 3.34 Å, inclusive (3.35Å≦d₍₀₀₂₎≦3.34 Å), preferably 3.354 Å to 3.370 Å, inclusive (3.354Å≦d₍₀₀₂₎≦3.370 Å; a crystallite size (L_(c)) in the c-axis directionobtained by X-ray diffraction of at least 200 Å, inclusive (L_(c)≧200Å), preferably between 200 Å and 1,000 Å, inclusive (200 Å≦L_(c)≦1,000Å); an average particle diameter (P_(d)) of between 1 μm to 30 μm,inclusive (1 μm≦P_(d)≦30 μm); a specific surface (SA) area of between0.5 m²/g to 50 m²/g, inclusive (0.5 m²/g≦SA≦50 m²/g); and a true density(ρ) of between 1.9 g/cm³ to 2.25 g/cm³ inclusive (1.9 g/cm³≦ρ≦2.25g/cm³).

Referring again to FIG. 1, to ensure that the electrodes 16,18 do notcome into electrical contact with one another, in the event theelectrodes 16,18 become offset during the winding operation duringmanufacture, the separator 20 “overhangs” or extends a width “a” beyondeach edge of the negative electrode 18. In one embodiment, 50 μm≦a≦2,000μm. To ensure alkali metal does not plate on the edges of the negativeelectrode 18 during charging, the negative electrode 18 “overhangs” orextends a width “b” beyond each edge of the positive electrode 16. Inone embodiment, 50 μm≦b≦2,000 μm.

The cylindrical casing 14 includes a cylindrical body member 30 having aclosed end 32 in electrical communication with the negative electrode 18via one or more negative electrode leads 34, and an open end defined bycrimped edge 36. In operation, the cylindrical body member 30, and moreparticularly the closed end 32, is electrically conductive and provideselectrical communication between the negative electrode 18 and anexternal load (not illustrated). An insulating member 38 is interposedbetween the spirally coiled or wound electrode assembly 12 and theclosed end 32. The insulating member 38 may be provided with apertures(not illustrated) for pressure equalization and/or electrolytepermeation.

A positive terminal subassembly 40 in electrical communication with thepositive electrode 16 via a positive electrode lead 42 provideselectrical communication between the positive electrode 16 and theexternal load (not illustrated). Preferably, the positive terminalsubassembly 40 is adapted to sever electrical communication between thepositive electrode 16 and an external load/charging device in the eventof an overcharge condition (e.g. by way of positive temperaturecoefficient (PTC) element), elevated temperature and/or in the event ofexcess gas generation within the cylindrical casing 14. Suitablepositive terminal assemblies 40 are disclosed in U.S. Pat. No. 6,632,572to Iwaizono, et al., issued Oct. 14, 2003; and U.S. Pat. No. 6,667,132to Okochi, et al., issued Dec. 23, 2003. A gasket member 44 sealinglyengages the upper portion of the cylindrical body member 30 to thepositive terminal subassembly 40.

A non-aqueous electrolyte (not shown) is provided for transferring ioniccharge carriers between the positive electrode 16 and the negativeelectrode 18 during charge and discharge of the electrochemical cell 10.The electrolyte includes a non-aqueous solvent and an alkali metal saltdissolved therein. Suitable solvents include: a cyclic carbonate such asethylene carbonate, propylene carbonate, butylene carbonate or vinylenecarbonate; a non-cyclic carbonate such as dimethyl carbonate, diethylcarbonate, ethyl methyl carbonate or dipropyl carbonate; an aliphaticcarboxylic acid ester such as methyl formate, methyl acetate, methylpropionate or ethyl propionate; a .gamma.-lactone such asγ-butyrolactone; a non-cyclic ether such as 1,2-dimethoxyethane,1,2-diethoxyethane or ethoxymethoxyethane; a cyclic ether such astetrahydrofuran or 2-methyltetrahydrofuran; an organic aprotic solventsuch as dimethylsulfoxide, 1,3-dioxolane, formamide, acetamide,dimethylformamide, dioxolane, acetonitrile, propylnitrile, nitromethane,ethyl monoglyme, phospheric acid triester, trimethoxymethane, adioxolane derivative, sulfolane, methylsulfolane,1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone a propylenecarbonate derivative, a tetrahydrofuran derivative, ethyl ether,1,3-propanesultone, anisole, dimethylsulfoxide and N-methylpyrrolidone;and mixtures thereof. A mixture of a cyclic carbonate and a non-cycliccarbonate or a mixture of a cyclic carbonate, a non-cyclic carbonate andan aliphatic carboxylic acid ester, are preferred.

Suitable alkali metal salts include: LiClO₄; LiBF₄; LiPF₆; LiAlCl₄;LiSbF₆; LiSCN; LiCl; LiCF₃ SO₃; LiCF₃CO₂; Li(CF₃SO₂)₂; LiAsF₆;LiN(CF₃SO₂)₂; LiB₁₀Cl₁₀; a lithium lower aliphatic carboxylate; LiCl;LiBr; Lil; a chloroboran of lithium; lithium tetraphenylborate; lithiumimides; sodium and potassium analogues of the aforementioned lithiumsalts; and mixtures thereof. Preferably, the electrolyte contains atleast LiPF₆.

Referring to FIG. 2, in another embodiment, a polymer laminate-typesecondary electrochemical cell 50 having an electrode active materialrepresented by general formula (I), includes a laminated or polymerstacked cell structure, having a negative electrode 52, a positiveelectrode 54, and an electrolyte/separator 56 there between. Thenegative electrode 52 includes a current collector 60 (preferably, acopper foil or grid) in electrical communication with a negativeelectrode membrane or film 62; and the positive electrode 54 includes acurrent collector 58 (preferably, an aluminum foil or grid) inelectrical communication with a positive electrode membrane or film 64.Protective bagging material 66 covers the cell and prevents infiltrationof air and moisture. Such structures are disclosed in, for example, U.S.Pat. No. 4,925,752 to Fauteux et al; U.S. Pat. No. 5,011,501 to Shackleet al.; and U.S. Pat. No. 5,326,653 to Chang; all of which areincorporated by reference herein.

The relative weight proportions of the components of the positiveelectrode 54 are generally: about 50-90% by weight active materialrepresented by general formula (I); 5-30% carbon black as the electricconductive diluent; and 3-20% binder chosen to hold all particulatematerials in contact with one another without degrading ionicconductivity. Stated ranges are not critical, and the amount of activematerial in an electrode may range from 25-95 weight percent. Thenegative electrode 52 includes about 50-95% by weight of a preferredintercalation material, with the balance constituted by the binder. In apreferred embodiment, the negative electrode intercalation material isgraphite. For test purposes, test cells are often fabricated usinglithium metal electrodes.

Those skilled in the art will understand that any number of methods areused to form films from the casting solution using conventional meterbar or doctor blade apparatus. It is usually sufficient to air-dry thefilms at moderate temperature to yield self-supporting films ofcopolymer composition. Lamination of assembled cell structures isaccomplished by conventional means by pressing between metal plates at atemperature of about 120-160° C. Subsequent to lamination, the batterycell material may be stored either with the retained plasticizer or as adry sheet after extraction of the plasticizer with a selectivelow-boiling point solvent. The plasticizer extraction solvent is notcritical, and methanol or ether are often used.

Separator membrane element 56 is generally polymeric and prepared from acomposition comprising a copolymer. A preferred composition is the 75 to92% vinylidene fluoride with 8 to 25% hexafluoropropylene copolymer(available commercially from Atochem North America as Kynar FLEX) and anorganic solvent plasticizer. Such a copolymer composition is alsopreferred for the preparation of the electrode membrane elements, sincesubsequent laminate interface compatibility is ensured. The plasticizingsolvent may be one of the various organic compounds commonly used assolvents for electrolyte salts, e.g., propylene carbonate or ethylenecarbonate, as well as mixtures of these compounds. Higher-boilingplasticizer compounds such as dibutyl phthalate, dimethyl phthalate,diethyl phthalate, and tris butoxyethyl phosphate are particularlysuitable. Inorganic filler adjuncts, such as fumed alumina or silanizedfumed silica, may be used to enhance the physical strength and meltviscosity of a separator membrane and, in some compositions, to increasethe subsequent level of electrolyte solution absorption.

Electrolyte solvents are selected to be used individually or inmixtures, and include dimethyl carbonate (DMC), diethylcarbonate (DEC),dipropylcarbonate (DPC), ethylmethylcarbonate (EMC), ethylene carbonate(EC), propylene carbonate (PC), butylene carbonate, lactones, esters,glymes, sulfoxides, sulfolanes, and mixtures thereof. The preferredsolvents are EC/DMC, EC/DEC, EC/DPC and EC/EMC. The salt content rangesfrom 5% to 65% by weight, preferably from 8% to 35% by weight. Oneexample is a mixture of EC:DMC:LiPF₆ in a weight ratio of about60:30:10. Desirable solvents and salts are described in U.S. Pat. Nos.5,643,695 to Barker et al. and 5,418,091 to Gozdz et al.

Examples of forming laminate and polymer stacked cells are disclosed inU.S. Pat. No. 4,668,595 to Yoshino et al.; U.S. Pat. No. 4,830,939 toLee et al.; U.S. Pat. No. 4,935,317 to Fauteux et al.; U.S. Pat. No.4,990,413 to Lee et al.; U.S. Pat. No. 4,792,504 to Schwab et al.; U.S.Pat. No. 5,037,712 to Shackle et al.; U.S. Pat. No. 5,262,253 toGolovin; U.S. Pat. No. 5,300,373 to Shackle; U.S. Pat. No. 5,435,054 toTonder et al.; U.S. Pat. No. 5,463,179 to Chalonger-Gill et al.; U.S.Pat. No. 5,399,447 to Chalonger-Gill et al.; U.S. Pat. No. 5,482,795 toChalonger-Gill and U.S. Pat. No. 5,411,820 to Chalonger-Gill; each ofwhich is incorporated herein by reference in its entirety. Note that theolder generation of cells contained organic polymeric and inorganicelectrolyte matrix materials, with the polymeric being most preferred.The polyethylene oxide of U.S. Pat. No. 5,411,820 is an example. Moremodern examples are the VdF:HFP polymeric matrix. Examples of casting,lamination and formation of cells using VdF:HFP are as described in U.S.Pat. No. 5,418,091 to Gozdz; U.S. Pat. No. 5,460,904 to Gozdz; U.S. Pat.No. 5,456,000 to Gozdz et al.; and U.S. Pat. No. 5,540,741 to Gozdz etal.; each of which is incorporated herein by reference in its entirety.

The following non-limiting examples illustrate the compositions andmethods of the present invention.

Example 1

An electrode active material of formula LiVOPO₄, was made as follows. Atroom temperature, 4.438 g (0.5 moles) of V₂O₃ (commercially availablefrom Stratcor) was added to 2 equivalents plus 10% of HCl to form asolution. Subsequently, 2 equivalents plus 10% of HNO₃ were slowly addedto the solution, and the solution was diluted with water to 100milliliters (mL) to yield a dark blue solution. 12.310 g (1 mole) ofLiH₂PO₄ as a 20% solution in water was added to the diluted dark bluesolution, followed by 3 equivalents of LiOH, yielding a solution havinga pH of 4.5. The resulting solution was stirred at room temperature,yielding a dark green precipitate. Thereafter, the solution was boiledto concentrate the solution to about 100 mL. The concentrated solutionwas filtered and washed three times, each time with 150 mL of water,yielding a dark green cake. The cake was dehydrated at 250° C. in airfor 1 hour, yielding 7.4 g of a dark green powder.

CuKα (λ=1.5405 Å) powder x-ray diffraction patterns were collected forthe as-made dehydrated precipitate material and the electrochemicallydelithiated material. The pattern shown in FIG. 3 indicates theprecipitate material to be α-LiVOPO₄, in fair agreement with thestructural analysis for α-LiVOPO₄ described by Journal of Solid StateChemistry, vol. 177 (2004), pgs. 2896-2902. The pattern for theelectrochemically delithiated material was not in agreement withα-VOPO₄; however, this is believed to be due to intercalation of solventinto the material.

Electrodes were formulated from a slurry with a weight ratio of 77:10:13of active material synthesized per this Example: Super-P carbon:PVDFbinder, respectively, which were mixed in a NMP-based slurry. The slurrywas cast onto aluminum foil, dried to remove the solvent and cut to forman electrode coupon. A cell was assembled using the coupon as a cathode,lithium metal as the anode, LiPF₆ in 2:1 ethylene carbonate:dimethylenecarbonate (EC:DMC) as the electrolyte and glass fiber as the separator.

FIG. 4 shows the cell cycled at constant current between 3.2 V and 4.4 Vfor one and a half cycles at a C/12 rate. The voltage curves for thecharge and discharge scans are similar in shape. The cell exhibited adischarge capacity of 77 mAh/g (milli-amp hours per gram), which waslower than expected and is believed to be due to the electrodeformulation. After the final half cycle, the cathode coupon was removedfrom the cell in the fully charged state and the XRD pattern was scannedas previously described.

Example 2

An electrode active material of formula LiVOPO₄, was made substantiallyper the teachings of Example 1. At room temperature, 4.628 g (0.5 moles)of V₂O₃ (commercially available from Stratcor) was added to a mixture of2 equivalents of HCl and 4 equivalents of HNO₃ to form a solution.12.838 g of LiH₂PO₄ as a 30% solution in water was added to thesolution. 20 mL of 30% H₂O₂ in water was slowly added, after which anaqueous solution of 8% LiOH monohydrate was added until the pH reachedabout 5.0. The resultant precipitate was then filtered and washed threetimes with 140 mL of water. The filter cake was then dried at 70° C.under partial vacuum for 15 minutes. The as-dried material was thendehydrated in air at 250° C. for 1 hour.

Referring to FIG. 5, CuKα (λ=1.5405

) powder x-ray diffraction patterns were collected for the as-driedprecipitate and the dehydrated material.

Electrodes were formulated from a slurry with a weight ratio of 77:10:13of active sample:super-p carbon:PVDF binder, which were mixed in aNMP-based slurry. The slurry was cast onto aluminum foil, dried toremove the solvent and cut to form an electrode coupon. A cell wasassembled using the coupon as a cathode, lithium metal as the anode,LiPF₆ in 2:1 EC:DMC as the electrolyte and glass fiber as the separator.

High-resolution electrochemical measurements were performed using theElectrochemical Voltage Spectroscopy (EVS) technique. EVS is a voltagestep method, which provides a high-resolution approximation to the opencircuit voltage curve for the electrochemical system underinvestigation. Such technique is known in the art as described by J.Barker in Synth. Met 28, D217 (1989); Synth. Met. 32, 43 (1989); J.Power Sources, 52, 185 (1994); and Electrochemica Acta, Vol. 40, No. 11,at 1603 (1995).

FIG. 6 shows an EVS scan of the material made by this means. There aretwo distinct reversible processes, at about 3.9V and 3.75V on discharge.The specific discharge capacity for this cell was calculated to be 112.8mAh/g.

The examples and other embodiments described herein are exemplary andnot intended to be limiting in describing the full scope of compositionsand methods of this invention. Equivalent changes, modifications andvariations of specific embodiments, materials, compositions and methodsmay be made within the scope of the present invention, withsubstantially similar results.

1. A method for making a compound represented by the general formula:A_(a)(VO)_(b)XO₄, wherein: (a) A is an alkali metal or mixture of alkalimetals and 0<a≦2; (b) X is selected from the group consisting ofphosphorous (P), sulfur (S), arsenic (As), silicon (Si), andcombinations thereof; and (c) A, X, a and b are selected to maintain theelectroneutrality of the compound; the method comprising the steps of:forming an acidic aqueous solution comprising vanadyl ions, ions of analkali metal, and ions of XO₄, wherein the step of forming the acidicaqueous solution comprises the step of dissolving one or more vanadylprecursor compounds, one or more alkali metal precursor compounds, andone or more precursor compounds containing moiety XO₄, in an aqueoussolvent; wherein the aqueous solution has a pH≦5 at standard temperatureand pressure; wherein: the one or more vanadyl precursor compounds isselected from the group consisting of V₂O₅, V₂O₃, VOSO₄, VOC₂O₄, andmixtures thereof; the one or more alkali metal precursor compounds andthe one or more precursor compounds containing moiety XO₄ are the samecompound and is selected from the group consisting of A₂SO₄, A₄SiO₄,AH₂PO₄, A₃PO₄, HAsA₂O₄, and mixtures thereof, wherein A is an alkalimetal; and precipitating the compound from the acidic aqueous solution.2. The method of claim 1, wherein 0≦pH≦4 at standard temperature andpressure.
 3. The method of claim 2, wherein 2≦pH≦4.
 4. The method ofclaim 1, wherein the step of dissolving one or more vanadyl precursorcompounds, one or more alkali metal precursor compounds, and one or moreprecursor compounds containing moiety XO₄, in an aqueous solvent, is asingle step dissolution reaction.
 5. The method of claim 1, wherein thestep of dissolving one or more vanadyl precursor compounds, one or morealkali metal precursor compounds, and one or more precursor compoundscontaining moiety XO₄, in an aqueous solvent, further comprises the stepof adding an acidic buffer in an amount sufficient to adjust the pH to≦5.
 6. The method of claim 1, wherein A is selected from the groupconsisting of Li, Na, K, and mixtures thereof.
 7. The method of claim 1,wherein A is Li.
 8. The method of claim 1, wherein the compound isLiVOPO₄.